Querying MDX Content via GraphQL

MDX files sourced via gatsby-source-filesystem can be queried with
allMdx on the root query. Like gatsby-transformer-remark, gatsby-mdx
adds fields to the Mdx node including excerpt, headings,
timeToRead, and wordCount.

TODO: insert full GraphQL MDX Node Schema

All static exports – values that would be valid JSON – are queryable
through the exports field.